WO2000044676A1

WO2000044676A1 - Novel method for filtering surface water with thin supernatant layer and implementing plant

Info

Publication number: WO2000044676A1
Application number: PCT/FR2000/000006
Authority: WO
Inventors: Laurent Probst; Thierry Coulom; Stéphane BOEGLIN
Original assignee: Centre International De L'eau De Nancy - Nan.C.I.E.
Priority date: 1999-01-27
Filing date: 2000-01-05
Publication date: 2000-08-03
Also published as: AU3050500A; EP1152984A1; US6685835B1; FR2788702B1; EP1152984B1; OA11824A; ATE263738T1; FR2788702A1; DE60009668D1

Abstract

The invention concerns a filtering method which consists in filtering the flow (Q) of the liquid to be filtered brought into contact with the surface (S2) of a filtering medium before passing right through said medium. The invention is characterised in that it consists in: generating at the surface of the filtering medium a thin liquid supernatant layer (e) by controlling its top level with appropriate means; producing in said thin layer a hydraulic flow path of length (L); bringing a liquid flow to be treated having a flow rate (Q) to the intake of said hydraulic flow path; recuperating said liquid flow with flow rate (nQ) which has circulated through the medium after n cycles and re-injecting it into the supernatant layer at the outlet of the hydraulic flow path, thereby producing in a section S1 of the flow path the meeting of two opposite flows (FE) and (FR); extracting after n cycles a treated liquid of flow rate (Q). The invention is applicable to surface water filtering.

Description

New process for filtering surface water with a thin supernatant layer and installation for implementation

The present invention relates to a new process for filtering liquid media, in particular but not limited to surface water, it also relates to the installations for implementing the process. A process traditionally used to treat surface water is slow biological filtration. which is a reproduction of the natural process of self-purification of water This technique consists in percolating a flow of water to be treated through a bed of filtering material at speeds below 10m / day (see principle diagram in figure A) 10 The functioning of such an installation must include a filtering material

(a) whose initial thickness is approximately one meter and a height of supernatant water

(b) from 1.5 to 2 meters. Consequently, the height of such a structure is between 2.5 and 3 meters.

A drainage system collects the filtered water at the bottom of the installation

In such an installation, the flow of suspended solids of the raw water entering the filter material (a) is distributed uniformly over the surface thereof, causing a uniform clogging of the entire filter surface and causing a raising the height of the supernatant water (b) so as to maintain a constant flow of treated water After the water has reached a level

20 maximum, it is necessary to intervene in order to operate an unclogging process. This unclogging is carried out in three main phases: emptying of the supernatant water, extraction of the first four centimeters clogged, filling of the filter with the water to be treated. These three operations are cumbersome in their implementation and require a fairly long time which can exceed one week. So is it

It is necessary to duplicate the installations in order to maintain the production of drinking water. Also, traditional slow biological filtration requires raw water of quality less than 10 NTU in order to limit over time the unclogging operations. Furthermore, the extraction of filter material during successive unclogging results in a gradual decrease in the height of this

30 last After reaching a height limit of 40 to 50 cm, it is necessary to restock with filter material, which is a limiting factor for the management of the installation.

The object of the invention is to overcome all of these constraints of the prior art. To achieve this goal, the applicant went against conventional wisdom and observed, under certain operating conditions, a new and unexpected phenomenon.

The recycling of an outgoing flow, as it is traditionally implemented, consists of reinjecting the outgoing flow into the inlet flow upstream of a facility, this leading to a dilution of the inlet flow.

This traditional recycling does not lead to improvements. The inventive idea consisted in reinjecting the outgoing flow on the surface of the filtering medium, by opposing it to the inlet flow and maintaining a low height of supernatant water, so that there is no flow dilution.

There is then a deposit of materials located on the surface of the filter material, deposit first near the inlet of the installation and then gradually enlarges towards the outlet until covering the entire surface of the filter material.

The new method according to the invention is a method for filtering liquid media, in particular but not limited to surface water, method according to which the liquid to be filtered of flow rate (Q) is brought into contact with the surface of a filtering medium. then crosses said medium right through, characterized in that a supernatant layer of thin liquid (e) is created on the surface of the filtering medium by controlling its high level by an appropriate means, - this thin supernatant layer is organized a hydraulic path of length (L),

- a flow of liquid to be treated of flow (Q) is brought to the inlet of the hydraulic path,

- the flow liquid (nQ) having passed through the filtering medium is recovered after n cycles by reinjecting it into the supernatant layer at the outlet of the hydraulic path, thus producing in a section of the path the meeting of two opposite flows,

- after n cycles, a treated flow rate liquid (Q) is extracted.

The e / L ratio is low and preferably less than 0.1. According to a variant, the high level of the supernatant layer of thickness (e) is determined by the position of the outlet of the treated flow liquid (Q).

According to another variant, the thickness (e) is limited by the choice of a filter medium in the form of a tube with a small internal diameter.

An installation for implementing the process is a filtration installation, in particular but not limited to surface water, installation comprising a filtering medium on the surface of which is brought a liquid to be treated with flow (Q), the latter passing through said media right through, installation characterized in that it comprises: means for determining on the surface of the filtering medium a layer of thin supernatant liquid (e), - means to determine in this thin layer a hydraulic path of length (L), an inlet for the liquid to be treated located at one end of said hydraulic path and characterized in that a recycling loop conducts the liquid from its exit from the filter medium to an injection point located at the other end of the hydraulic path where the outlet of the filtered liquid is also located at the outlet of the installation.

The new process according to the invention alleviates all the drawbacks of the prior art, and the installations which implement it are of simple design, their main advantages are the following: height of the works considerably reduced (height divided by 6) capacity of treat high loads of suspended matter, for example: surface water, karstic water with turbidity> 300 NTU capacity to maintain a quality of treatment despite a strong variation in the quality of raw water

- no addition of chemical reagent organization of clogging and its limitation on the surface without penetration (stacking of suspended matter in the first part of the filter) obtaining clogging only on the surface - direct access to the clogged layer from above the unclogging filter very easy requiring a squeegee or water jet type tool

- have no loss and no extraction of filter material hence the sustainability of the installation

- keep an intense biological activity (because the very sown filter material on the first centimeters is not extracted after each washing of the clogged layer)

- very low energy consumption (less than 0.4 kw / m3 of produced water).

According to an installation variant, the filtering medium is sand and the high level of the supernatant layer is determined by the level of the outlet of the filtered liquid at the outlet of the installation. According to another variant, the filter medium is a set of filter tubes with membrane walls traversed by the liquid and in that the thickness (e) is limited by the diameter of the tubes.

The invention will be better understood using the following description made with reference to the following appended figures:

Figure A: sketch showing, seen in vertical section, an installation of the prior art,

Figure 1: sketch showing, seen in vertical section, a non-limiting example of installation according to the invention and its operating principle, - Figure 2: sketch in top view of the installation of Figure 1,

- Figure 3: perspective sketch of the installation of Figure 1 showing only its operation,

Figure 4: detail of the clogging area in the installation of Figure 1, Figure 5: sketch illustrating the washing process of the installation of Figure 1, - Figure 6: sketch showing a top view of a path hydraulic can be provided in the supernatant blade of the installation, Figures 7a and 7b: sketch showing, seen in section, a membrane filtration module of the prior art, operating according to two known methods,

- Figure 8: sketch showing, seen in section, a membrane filtration module operating according to the method of the invention.

Figures 9a and 9b: sketch showing a top view and vertical sectional view of a variant of double-drainage filtration tank

First, a non-limiting example describes an installation operating according to the method of the invention. The filtration installation shown in FIGS. 1 to 5 comprises a filtration tank whose general shape is parallelepipedic with two side walls

(4,5) preferably larger than the two end walls (2,3). At one end, a vertical partition wall (6) is provided, parallel to the inlet end wall (2), and preferably of the same width (λ) so as to create an inlet compartment (8). into which the raw water inlet (9) opens.

The partition wall (6) extends to the bottom of the tank (1) to allow washing which will be explained later, and its height is less than that of the end wall (2) so as to allow, after filling the compartment (8), a continuous and regular passage of the raw water above said separation wall (6) and over the entire width (λ).

The raw water thus arrives horizontally on the free surface of a filtering medium (10), here sand, contained in the main tank of the installation (11) passes through the layer of filtering medium (10), then is extracted by a pump (13) through one or more drains (12) provided at the bottom of the tank and reinjected into an outlet compartment (14) for at least recycling under the conditions below.

The outlet compartment (14) is separated from the main tank (11) by a partition wall (15) parallel to the outlet wall (3) of the main tank (11) and of the same width (λ) as said tank (11 ).

The water from the drains is injected into the outlet compartment (14) and preferably at the bottom.

Said outlet compartment does not necessarily have the same depth as the main tank (11) but the horizontal upper edge of the partition wall (15) is necessarily at the same level as that of the partition wall (6) so as to that the recycled water, after filling the outlet compartment (14) goes to the main tank, over the wall (15) and is distributed evenly over the free surface of the filter medium (10).

The two flows of raw water (F _E ) and recycled water (F _R ) flow in opposite directions above the surface (S ₂ ) of the filter medium and meet at a fictitious contact surface (S ^ vertical They are generally symbolized by arrows (FE) and (F _R ) in opposite directions, on either side of said contact surface (S.

The level of liquid above the free surface of the filter medium is determined by the position of the outlet (16) of treated water located in the outlet compartment. The thickness of this layer of supernatant liquid will be designated by (e). From the first recycling of water, the flow (F _R ) of recycled water, already filtered and clean, opposes the flow (F _E ) of water to be treated and dirty and very quickly, we observe that the raw water remains confined to the inlet end of the main tank. As the recycling progresses, it is surprisingly found that the materials are not retained in the filtering medium but that a layer of sedimented material (17) is created, a few millimeters in surface, which gradually extends towards the outlet, and gradually clogs the free surface of the filter medium.

Overall, it can be said that the contact surface (S ^ between the two flows moves progressively from the inlet to the outlet of the installation. The clogging of the filter media induces, for a constant pumping force, an increase in losses load and ultimately a decrease in the recycling rate.

As long as the recycling rate is higher than the inlet rate, the quality of the water is substantially constant because the reduction in filtration cycles is offset by the filtration efficiency of each cycle which increases as the clogging.

The operating conditions of the installation, taking into account the parameters indicated in FIG. 3, are as follows: e = thickness of the supernatant layer S ₂ = horizontal projection of the upper surface of the filtering medium

If = contact surface of the flows F _E and F _R it is necessary:

S ₂ >> Sι either L x λ>> ex λ or L>> e or e / L <<1 therefore an e / L <<1 ratio is necessary and preferably but not limited to less than 0.05.

At each start of the installation, after unclogging, the operating equations of the installation are written as follows:

• for a given flow rate Q of raw water at the inlet (9) and of treated water at the outlet (16) and for a flow rate (n-1) x Q of recycled water and reinjected into the main tank (11 );

• with L = length of the path X = length when the raw water is clogged

Y = L- X = length not subject to clogging of raw water n> 1, n = recycling rate

• the propagation of the front (Si) results in the equations X = / nx L and Y = L x (1 - 1 / n). Thus, when the number n of cycles increases, X increases and Y decreases. The speeds at the entry and exit of the installation, adjusted for a given dimensioning of the installation, produce at entry and exit flows of turbulent regimes remaining confined in the entry (8) and exit ( 14). Their speed decreases during their advancement in the supernatant blade (e). A laminar flow is obtained on either side of the contact surface (SL), thus limiting chemical and physical exchanges of all kinds between the two fluids as much as possible.

For e / L <0.01 the best operating results are obtained and the flows (F _E ) and (F _R ) do not mix.

For 0.01 <e / L <0.05 we still get very good results.

For 0.05 <e / L <0, 1 the results are acceptable. For increasing values of e / L and greater than 0, 1, there is gradually observed an increasingly large mixture between the flows (F _E ) and (F _R ) detrimental to the proper functioning of the installation without, however, to prohibit.

We now explain the washing methods of the installation (see Figure 5)

A first surface washing is carried out, either regularly or when it is noticed that the layer (17) of sediment almost completely covers the free surface of the filtering medium. For this, the installation is stopped beforehand by closing the water inlet to be treated (9) and the recycled water outlet (13).

Using a jet (18) and / or a squeegee, the surface sediments are pushed towards the inlet compartment and are discharged through the washing outlet (7) located at the bottom of the inlet compartment. (8). This operation is very simple and very fast. The filtered materials having remained above the filtering medium without mixing with it, washing according to the invention will only involve the materials deposited without reducing the height of the filtering medium (no loss of sand), unlike the installations of the prior art where the materials retained and the sand are mixed (loss of sand by extraction of the mixture to unclog).

Optionally, provision may be made to use treated water stored by pumping (21) in a tank (20) as washing water, as illustrated in FIG. 5. So that there is no clogging of the filter media in its thickness, following for example an accidental malfunction, it is possible to carry out a deep washing by deep injection of a pressurized liquid through a tube (19) introduced and moved into the mass of sand. Alternative embodiments can be provided, for example by using two or more filtration tanks such as (1) in series, if the quantity of material to be separated is large (variant not shown).

It is also possible to provide for directing the flow flows (F _E ) and (F _R ) on the free surface of the media using corridors as for example in FIG. 6.

The length (L) is in this case the length of the total flow path between the entrance and exit of the corridor. This construction allows for an equal surface, to lengthen (L) therefore further reducing the e / L ratio and optimizing the installation.

According to another variant of the sand filter shown in FIGS. 9a and 9b, the installation can be fitted with double drainage. An upper drainage (28) whose geometry follows that of the flow path in the chicane corridors arranged in the supernatant blade (e), said drainage being placed in the upper part of the filtering medium, and it is characterized by a high flow (n = 10) which minimizes the length X. Preferably, the outlet (29) of this drain is on the side of the raw water inlet (9) to be recycled at the outlet (14) of the installation. A lower drainage (12) is arranged at the bottom of the filtering medium as in FIGS. 1 to 6, it is characterized by a much lower flow rate (n = 1, 5) which makes it possible to maintain a slow filtration speed.

In all of the above, it will be understood that the sand which constitutes the filtering medium can be composed of a layer of sand of homogeneous particle size or of several superimposed layers of sand of different particle sizes, for example two layers (31, 32) on the Figure 9b.

It is also possible to use other types of filtration means such as membrane filter modules known and used in micro, ultra-filtration and nano-filtration.

By way of example, there is shown such a module operating in frontal mode in FIG. 7a, in tangential mode in FIG. 7b (two known operating modes).

Such a module comprises several bundles of filter tubes (22), the walls of which constitute the filter medium through which the liquid to be treated passes. In the examples of FIGS. 7a and 7b, the liquid to be treated (23) is injected inside the tubes (22) and collected after filtration in the interstitial interval

(24) then extracted by two outlets (25) from the metal casing (26).

In the case of FIG. 7b, an untreated fraction (27) of the liquid is reinjected into the tubes for a traditional recycling operation as described in the introduction to the present application.

FIG. 8 shows the same module operating according to the new principle of the invention.

The flow liquid to be filtered (Q) is injected at one end and inside the tubes, crosses their walls, then is extracted at the outputs (25) from the module casing and partially reinjected inside the tubes but at the end opposite the inlet end of the liquids to be treated. . .

Thus, one obtains in each tube two opposing flows (F _E) and (F _R ) meeting in a section (Si) of the tube, and a deposit of sediment on the inner wall of the tube, gradually extending to clutter the total length (L) of the tube. The cleaning will then be easily carried out by injection of liquid under pressure after stopping the device.

The ratio e / L and here determined by the dimensions of the components: (e) being the diameter of a tube and L its length.

Claims

1. Method for filtering liquid media, in particular but not limited to surface water, method according to which the liquid to be filtered with flow rate (Q) is brought into contact with the surface (S ₂ ) of a filtering medium and then passes through said filter media through and through, characterized in that

a supernatant layer of thin liquid (e) is created on the surface of the filtering medium by controlling its high level by an appropriate means,

- a hydraulic path of length (L) is organized in this thin layer,

- a flow of liquid to be treated with flow (Q) is brought to the inlet of the hydraulic path, - the flow liquid (nQ) having passed through the filter medium is recovered after n cycles by reinjecting it into the supernatant layer at the outlet of the hydraulic path, thus producing in a section S1 of the path the meeting of two opposite flows (F _E ) and (F _R ),

- after n cycles, a treated flow rate liquid (Q) is extracted.

2. A filtration method according to claim 1, characterized in that an e / L ratio of less than 0.1 is chosen.

3. A filtration method according to claim 2, characterized in that an e / L ratio of less than 0.05 is chosen.

4. A filtration method according to claim 3, characterized in that an e / L ratio less than 0.01 is chosen.

5. Filtration method according to one of claims 1 to 4, characterized in that the high level of the supernatant layer of thickness (e) is determined by the position of the outlet (16) of the treated flow liquid ( Q).

6. A filtration method according to one of claims 1 to 4, characterized in that the thickness (e) is limited by the choice of a filter medium in the form of a tube with a small internal diameter.

7. Filtration installation, in particular but not limited to surface water, installation comprising a filtering medium on the surface of which is brought a liquid to be treated with flow (Q), this passing through said medium right through, installation characterized in that it includes: - a means for determining on the surface of the filtering medium a layer of supernatant liquid of small thickness (e), means for determining in this thin layer a hydraulic path of length (L), - an inlet of the liquid to be treated located at a end of said hydraulic path and characterized in that a recycling loop conducts the liquid from its outlet from the filtering medium to an injection point located at the other end of the hydraulic path where the outlet (16) of the liquid filtered at the outlet of the installation.

8. Installation according to the preceding claim, characterized in that the filtering medium is sand and in that the high level of the supernatant layer is determined by the level of the outlet (16) of the filtered outlet liquid.

9. Installation according to claim 8, characterized in that the sand is contained in a main tank (11) of rectangular shape comprising at one of its ends an inlet compartment (8) into which opens a raw liquid inlet (9 ), at its other end an outlet compartment (14) which is located the outlet (16) of the treated liquid, the water having passed through the sand being extracted by at least one drain (12) then reinjected into the outlet compartment (14) for at least recycling.

10. Installation according to claim 9, characterized in that it comprises in the lower part of the inlet compartment, a washing outlet (7).

11. Installation according to one of claims 8 to 10, characterized in that it further comprises a reservoir (11) for storing washing water.

12. Installation according to one of claims 9 to 11, characterized in that it comprises an upper drainage (28) characterized by a large flow, and a lower drainage (12) characterized by a lower flow.

13. Installation according to claim 7, characterized in that the filtering medium is a set of filter tubes with membrane walls traversed by the liquid and in that the thickness (e) is limited by the diameter of the tubes.